The present invention relates to apparatus with which to control gas flows in vacuum furnaces, wherein the contents of the furnace are heated as well as cooled in a charge chamber, by means of a blower and gas circulation. Temperatures of up to about 750.degree. C. are contemplated in such furnaces.
As a rule such furnaces are composed of a cylindrical pressure-resistant housing capable of holding a heated load chamber therein enclosed by thermal insulation, together with a heat exchanger and a blower to circulate the heating and cooling gas. Advantageously during both the operational stage wherein the load is heated convectively and for the cooling cycle, the gas is guided through pipes into the load or work chamber, where said pipes act as heating elements. The pipes are mounted axially with respect to the surface of the cylindrical load chamber and are provided with nozzles pointing at the load. Further advantageously, the gas is conducted in such a way as to circulate during both operational stages by the same blower. A typical furnace is described in the German patent 37 36 502.
To make certain that the same gas may both heat and cool the contents of the chamber, the furnace requires a control system which allows switching the gas flow circulated by the blower between two circuits. In one circuit, the gas will circulate only within the furnace region equipped with thermal insulation; in the other circuit it will be guided over the heat-exchange pipes located between the thermal insulation and the receptacle wall.
In the furnace described in German patent 37 36 502, this problem is solved by integrating a box between the load chamber and the suction side of the blower inserted into a gas manifold device. The box is equipped with apertures both toward the load chamber and toward the annular space between the thermal insulation and the receptacle wall wherein the heat-exchange pipes are mounted. This box houses a slider means which can be displaced by a piston rod transversely to the furnace axis. Depending on the slider position, the apertures toward the load chamber or the annular space between the thermal insulation and receptacle wall are cleared, and the particular other apertures are simultaneously closed.
This design incurs the drawback that the slider can seal only apertures of small cross-sections, whereby high pressure losses follow when the gases flow through them. Moreover the flow to the blower takes place asymmetrically and as a result the gas flow is unevenly spread over the heating pipes. Another drawback is that the slider course between the two end positions is very long. Its actuation requires a very long cylinder which projects from the furnace housing and thereby restricts the applicabilities of such furnaces.